EMMANUEL Macron dealt a heavy blow to Iranian nuclear ambitions when he promised the state will “never possess any nuclear weapon” during a speech in the US Congress.

Macron called on Donald Trump not to abandon a nuclear agreement with Iran but warned Tehran that France remained committed to preventing any nuclear weapon development.

The French president said: “Iran shall never possess any nuclear weapon. Not now not in five years, not in 10 years, never.”

Capping a three-day visit to the United States in which he urged President Donald Trump to stay in the Iran pact, Macron told Congress the current deal was not perfect but must remain in place until a replacement was forged.

He said: "It is true to say that this agreement may not address all concerns and very important concerns.

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"But we should not abandon it without having something substantial and more substantial instead. That's my position."

French opposition leader Jean-Luc Mélenchon warned a war with Iran was “on the horizon”.

He tweeted: “Pathetic attempt by Macron to align himself with Trump. In Iran, war is now on the horizon. Macron [poses a] dangerous threat to peace.”

Trump has often vowed to pull the United States out of the 2015 nuclear pact between Iran and six major powers.

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World War 3: Macron promised Iran will never have nuclear weapons

Iran shall never possess any nuclear weapon

Emmanuel Macron

The US President will decide by May 12 whether to restore US economic sanctions on Tehran, which could be a first step to ending the deal.

US non-proliferation envoy Christopher Ford said on Wednesday: “We are not aiming to renegotiate the JCPOA (nuclear deal) or reopen it or change its terms.

"We are seeking a supplemental agreement that would in some fashion layer upon it a series of additional rules - restrictions, terms, parameters, whatever you want to call it - that help answer these challenges more effectively."

"If we were able to meet that challenge of bringing our partners together, President Trump made it clear that his decision not to renew the sanctions waivers would be revised, and that is where I hope we are now today.

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US President Donald Trump and French President Emmanuel Macron hugging during a joint press conference

"I hope the JCPOA has been saved in the context of the challenge that President Trump set for us, of trying to remain within the deal but in the context of moving forward with our partners on an approach that stands a pretty good chance of turning what was in effect a temporary postponement into a more enduring answer."

In a broad speech, Macron also urged US lawmakers to reject unilateralism and remain engaged with the world, saying modern economic and security challenges must be a shared responsibility.

He said: "The United States is the one who invented this multilateralism. You are the one now who has to help preserve and reinvent it.

"Let us work together in order to make our planet great again.

"I believe in building a better future for our children, which requires offering them a planet that is still habitable in 25 years."